Severe Combined Immunodeficiency Disease (SCID) is an autosomal recessive mutation that occurs in Athabascan speaking Indians (Navajo and Apache) at a frequency of approximately 2%. The incidence of SCID in Navajo babies is 1:2000 live births. The specific aim of this study is to map the "Navajo SCID" recessive gene by using genetic linkage, looking for coinheritance of the SCID gene with a DNA marker of known chromosomal location. To accomplish this, we will use a collection of highly polymorphic DNA markers including CA repeat microsatellite markers throughout the human genome. To date, we have over 200 DNA markers, including 80+ variable number tandem repeats (VNTR) and 125 CA repeat microsatellite marker. To study inheritance, we have assembled a collection of DNA preparations from 16 different Navajo kindreds. This includes 17 affected children and a total of 78 family members. The affected sample population is growing at approximately two kindreds per year. The DNA samples will be typed using the DNA probes and microsatellite markers. These data will be analyzed to find linkage between a mapped probe and the SCID gene. We plan to use the linkage information for prenatal diagnosis of affected fetuses and carrier detection of unaffected relatives. Eventually, we plan to identify and characterize the gene which causes SCID in this native American population and understand its role in regulating immune function.